Introduction

The DNA in eukaryotic cells is extensively folded and packed into the protein- DNA complex called chromatin. Histones proteins are an important part of this complex. DNA protein complex is called nucleosome. Walther Fleming discovered chromatin in 1878 while study cell division. He stained the cells of salamander with alanine dye. He observed that some portion of genetic material absorbed dye more than other parts. He named it chromatin.

Types of chromatin

Euchromatin: It is a loosely packed form of chromatin and rich in gene concentration. It is comprising the most active portion of genome It is truly transcribing portion of DNA. This portion is called “string on the beads”.

Heterochromatin: It is tightly packed form of the chromatin. This region of chromosome is more condensed and stained deeply with different dyes.

Chromatin structure

The DNA in eukaryotic cells is extensively folded and packed into the protein-DNA complex called chromatin. Histones are an important part of this complex since they both form the structures known as nucleosomes and contribute significantly into gene regulatory mechanisms.

Role of chromatin

• DNA packaging into more compact form.
• Reinforce the DNA macromolecules to allow mitosis.
• Prevent DNA from damaging.
• Regulation of gene expression.
• Regulation of DNA replication.
• Control the amount of gene product
• Allow cells to adjust with changing conditions
• Expression of gene at proper time.
• Prevent the wastage of cell energy.

Steps involved in gene regulation

Chromatin Remodeling:

Large regions of chromatin are transcriptionally inactive in some cells while they are either active or potentially active in other specialized cells

Formation and disruption of nucleosome structure

The presence of nucleosomes and of complexes of histones and DNA provide a barrier against the ready association of transcription factors with specific DNA regions.

Histone Acetylation and deacetylation

The amino-terminal tail of histone H3 extends into a pocket in which a lysine side chain can accept an acetyl group from acetyl CoA bound in an adjacent site.

Enhancers and Repressors

Enhancer elements are DNA sequences, although they have no promoter activity of their own, but they greatly increase the activities of many promoters in eukaryotes. Enhancers function by serving as binding sites for specific regulatory proteins.

Locus control regions and Insulators

Some regions are controlled by complex DNA elements called locus control regions (LCRs). Another mechanism is provided by insulators. These DNA elements, also in association with one or more proteins, prevent an enhancer from acting on a promoter.

Gene Amplification

The gene product can be increased by increasing the number of genes available for transcription of specific molecules. Among the repetitive DNA sequences are hundreds of copies of ribosomal RNA genes and tRNA genes

Gene Rearrangement

 Gene rearrangement is observed during immunoglobulins synthesis. Immunoglobulins are composed of two polypeptides, heavy (about 50 kDa) and light (about 25 kDa) chains.

Alternative RNA Processing

Eukaryotic cells also employ alternative RNA processing to control gene expression. This can result when alternative promoters, intron-exon splice sites, or polyadenylation sites are used.

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Journal of Molecular Oncology Research
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